Identification of foot and mouth disease virus carrier and subclinically infected animals and differentiation from vaccinated animals.

Countries that are free of foot and mouth disease (FMD) are reluctant to use vaccine in the event of an outbreak because of the difficulties this can cause in re-establishing freedom from FMD status to the satisfaction of trading partners. The problem does not lie in distinguishing between vaccinated and recovered animals as vaccinated animals can be tagged or otherwise marked to show that they have been vaccinated; the difficulty is in identifying vaccinated animals that have had contact with live virus and become carriers. The traditional probang test is not sufficiently sensitive and is labour- and laboratory-intensive, but alternative serological tests such as those for antibodies to non-structural proteins (NSPs), or specific immunoglobulin A (IgA) are also not 100% sensitive. However, these newer tests do provide increased security by reducing the likelihood of trading carrier animals and can be used to help define the limits of an outbreak; the use of vaccine to help control an outbreak of FMD in a previously free country still has significant consequences on trade in FMD susceptible animals and their products.